1. Field of the Invention
The present invention relates generally to the manufacture of urethane foam articles, and more particularly to a method of preparing urethane foam articles having a high density outer surface layer suitable for use as interior parts of an automobile, a ship, etc., or component parts of a furniture.
2. Description of the Prior Art
In the manufacture of urethane foam articles having a high density outer surface layer, it has been customarily practice to use a low boiling point chlorofluorocarbon blowing agent such as CFC-11 or CFC-113 together with a catalyst such as 1,4-diazabicyclo(2,2,2)octane or dibutyltin dilaurate.
During the reactions that produce a urethane foam, the heat of reaction is conducted to a metal mold, thereby creating a temperature difference in the foaming plastic mass. This temperature difference varies the vaporization rate of a low boiling point solvent (blowing agent) such as CFC-11 or CFC-113 which has been mixed with and homogeneously compatibilized in a urethane stock solution, so that an outer portion of the foaming plastic mass which contacts the mold surface forms a high density outer surface layer, while an inner portion of the foaming plastic mass which is held out of contact with the mold surface has a cellular structure. As the reaction advances, the mold cavity is filled with foams or bubbles and the blowing pressure in the mold rises. In this instance, gases trapped in those foams which are contacting the mold surface are condensed and then absorbed or recompatibilized in the reacting plastic mass. Thus, the high density outer surface layer is thickened. In this condition, the foaming plastic mass is forced into rapid gelation or polymerization to set the shape of the high density outer surface layer and the inner cellular layer. A urethane foam article having a foam core and a high density outer surface layer is thus formed by a single foam molding operation of the same urethane stock solution. The conventional manufacture of such urethane foam article having a high density outer surface layer involves the consumption of CFC-11 or CFC-113.
Since CFC-11 and CFC-113 are completely halogenated hydrocarbon blowing agents including fluorine and chlorine, the consumption of CFC-11 and CFC-113 causes a breakage or destruction of the ozone layer in the stratosphere which will permit a great amount of detrimental ultraviolet rays (especially UVB, 320-280 nm) to reach the ground of the earth. The detrimental ultraviolet rays bring about various serious problems such as the mutation, deactivation and damaging of genes and cells, an increased number of cutaneous cancer, a change of the ecosystem, and the greenhouse effect resulting from an increase in the atmospheric temperature.
With the foregoing problems in view, there has been an increased demand for a technique for the preparation of a urethane foam article having a high density outer surface layer either with the use of a reduced amount of CFC-11 or CFC-113 as a blowing agent, or without the use of CFC-11 or CFC-113. Such technique may include (1) a production using a low boiling point combustible solvent, (2) a substituted formation of a thick high density outer surface layer by the in-mold coating process, or (3) a production employing the gas-loading process using dry air or nitrogen gas.
The foregoing techniques are not satisfactory due to the possibility of an accidental firing of the combustible solvent, the necessity of an additional heating of a semi-finished product for removing the combustible gas, a high toxicity of the combustion gas and rust created thereby, and a high reconstruction cost of the manufacturing equipment. Furthermore, the high density outer surface layer formed by any of these techniques is not satisfactory in quality.